Avalanche photodiodes (APD) are commonly used as photodetectors to detect the presence of photons within an area. Generally, the crystalline materials forming an APD include a conduction band and a valence band. The conduction band and the valence band are separated by an energy level (Eg) commonly referred to as a bandgap. As shown in FIG. 1, when electromagnetic radiation equal to or greater than the energy level Eg is incident on the surface of the crystalline APD material some electrons in the valence band 1 absorb the incident energy and are activated across the bandgap 5 to the conduction band 3. An exemplary source of incident electromagnetic radiation may include photons. This intrinsic activation results in the generation of one electron in the conductor band 5 and one hole in the valence band 3 for each interaction with the incident energy.
FIG. 1 shows an electron 7 which, when exposed to incident energy 9, has been activated across the bandgap 5 thereby moving from the valence band 1 to the conduction band 3. Thereafter, the absorption mechanism reaches a dynamic equilibrium in conjunction with recombination. As shown in FIG. 1, eventually the electron 7 activated to the conduction band 3 reaches a state of equilibrium 11 within the conduction band 3. Eventually, the activated electron 7 will leave the conduction band 3 via a recombination mechanism and return to valence band 1, thereby returning the crystalline material to a static state of thermodynamic equilibrium. FIG. 1 shows the electron 7 reaching thermodynamic equilibrium and moving along a path 15 from the conduction band 3 to the valence band 1. As such, activated electron returns to a state of equilibrium within a time period of T1.
Often, the crystalline materials forming the APD include one or more irregularities or impurities therein which may affect the activation and relaxation process. FIGS. 1 and 2 show crystalline materials forming the APD having one or more irregularities or traps 13 therein. The trap 13 may prevent the electron 7 from immediately returning to a state of equilibrium until released by external stimuli (e.g. thermal agitation) or until a sufficient period of time passes. As shown in FIG. 2, some activated electrons return to the valence band 1 immediately, as illustrated by the direct path 15. However, some activated electrons encounter and are restrained from immediately returning to the valence band 1 by the trap 13. As such, the delayed electrons proceed on along a multiple paths 17A, 17B. Further, electrons encountering and restrained by the trap 13 will return to a state of equilibrium at a second time period of T2, wherein T2 is greater than T1. As such, photocurrents may continue to be generated within the APD after the application of photons to the APD has stopped. Typically, this phenomenon is referred to as dark current or detector noise. FIG. 3 shows an example of noise generated by traps within an APD. As shown, incident a photon on the APD generates a first photocurrent 19 within an electrical circuit coupled to the APD. The electrons delayed by the deep trap within the APD continue to generate secondary photocurrents 21 within the APD for a period following the cessation of the application of light to the APD.
When the APD is used to detect low levels of light the electrons delayed by the trap 13 may generate detector noise and/or dark current approximately equal to the amount of light incident on the APD. As such, the measuring accuracy of the APD may be inaccurate. Further, the responsiveness of the APD may be compromised, thereby requiring an extended period of time between measurement cycles to permit the detector to reach a state of equilibrium.
Thus, in light of the foregoing, there is an ongoing need for a system capable of rapidly detecting light at various levels.